The present invention relates to transmit-receive switching in a single-antenna time division control type FM-CW radar and, more particularly, to a means for providing reliable isolation between transmitting and receiving systems.
FM-CW radar is used as a radar system for measuring distance and relative velocity with respect to a target object. Since FM-CW radar can measure the distance and the relative velocity of a vehicle traveling in front by using a simple signal processing circuit, and as its transmitter and receiver can be constructed with simple circuitry, this type of radar is used as an automotive collision avoidance radar.
The principle of FM-CW radar is as follows. An oscillator is frequency-modulated, for example, by a triangular wave of several hundred hertz, and the frequency-modulated wave is transmitted; then, a reflected signal from a target object is received, and the received signal is frequency-demodulated using the frequency-modulated wave as the local frequency. Here, the reflected wave from the target is shifted in frequency from the transmitted signal (i.e., produces a beat) according to the distance between the radar and the target and also to the Doppler shift due to the relative velocity of the target. Accordingly, the distance and the relative velocity of the target object can be measured from this frequency shift.
In an FM-CW radar system, a triangular wave is often used as the modulating signal, and the description given herein deals with the case where a triangular wave is used as the modulating signal, but it will be appreciated that a modulating wave of other shape, such as a sawtooth wave or a trapezoidal wave, can be used instead of the triangular wave.
FIG. 1 is a diagram showing the configuration of a single-antenna FM-CW radar. As shown in FIG. 1, a modulating signal generator 1 applies a modulating signal to a voltage-controlled oscillator 2 for frequency modulation, and the frequency-modulated wave is passed through a circulator 5 and transmitted out from the antenna AT. The signal thus transmitted out and reflected from a target object, such as a vehicle traveling in front, is received by the antenna AT, and the received signal is input to a frequency converter 4 via the circulator 5. In the frequency converter 4, the input signal is mixed with a local signal separated by a directional coupler 3 from the output of the voltage-controlled oscillator 2, to produce a beat signal.
FIG. 2 is a diagram for explaining the principle of FM-CW radar when the relative velocity with respect to the target object is zero. In this case, the transmitted wave is a triangular wave whose frequency changes as shown by a solid line in part (a) of FIG. 2. In the figure, f0 is the center frequency of the transmitted wave, xcex94f is the FM modulation amplitude, and Tm is the repetition period. The transmitted wave is reflected from the target object and received by the antenna; the received wave is shown by a dashed line in part (a) of FIG. 2. The round trip time of the radio wave to and from the target object is given by T=2r/C, where r is the distance to the target object and C is the velocity of propagation of the radio wave.
Here, the received wave is shifted in frequency from the transmitted signal (i.e., produces a beat) according to the distance between the radar and the target object. The beat frequency component fb can be expressed by the following equation.
xe2x80x83fb=fr=(4xc2x7xcex94f/Cxc2x7Tm)rxe2x80x83xe2x80x83(1)
FIG. 3, on the other hand, is a diagram for explaining the principle of FM-CW radar when the relative velocity with respect to the target object is v. The frequency of the transmitted wave changes as shown by a solid line in part (a) of FIG. 3. The transmitted wave is reflected from the target object and received by the antenna; the received wave is shown by a dashed line in part (a) of FIG. 3. Here, the received wave is shifted in frequency from the transmitted signal (i.e., produces a beat) according to the distance between the radar and the target object. In this case, since the relative velocity with respect to the target object is v, a Doppler shift occurs, and the beat frequency component fb can be expressed by the following equation.
fb=frxc2x1fd=(4xc2x7xcex94f/Cxc2x7Tm)rxc2x1(2xc2x7f0/C)vxe2x80x83xe2x80x83(2)
The symbols in the above equations (1) and (2) have the following meanings.
fb: Transmit/receive beat frequency
fr: Range (distance) frequency
fd: Velocity frequency
f0: Center frequency of transmitted wave
xcex94f: Frequency modulation amplitude
Tm: Period of modulated wave
C: Velocity of light (velocity of radio wave)
T: Round trip time of radio wave to and from target object
r: Range (distance) to target object
v: Relative velocity with respect to target object
Thus, the distance and the relative velocity with respect to the target object can be obtained from the above equation (1) or (2).
Here, consider the beat signal power; then, the receiving power, Pr, of the antenna can be expressed by
Pr={(G2xc2x7xcex2xc2x7"sgr"xc2x7Pt)/((4xcfx80)3xc2x7r4)}xc2x7Laxe2x80x83xe2x80x83(3)
and the beat output power Pb by
xe2x80x83Pb=Prxc2x7Cmixxe2x80x83xe2x80x83(4)
The symbols in the above equations (3) and (4) have the following meanings.
G: Antenna gain
xcex: Wavelength
"sgr": Reflecting object area
Pt: Transmitting power
r: Range to target object
Cmix: Conversion loss in mixer
On the other hand, a single-antenna time division control type FM-CW radar uses a single antenna as a transmitting/receiving antenna AT, which is switched between transmission and reception by time division control with a transmit-receive switching section comprising a switching means.
However, in the case of the single-antenna time division control type FM-CW radar, since the timing is switched alternately between transmission and reception, the transmitting power may leak into the receiving system through the transmit-receive switching section. This can cause such problems as saturation gain compression of the receiving system, increased leakage of the transmit-receive switching frequency into the receiving system, and increased FM-AM conversion noise.
Accordingly, it is an object of the present invention to provide increased isolation between the transmitting and receiving systems in a single-antenna time division control type FM-CW radar and thereby prevent the transmitting power from leaking into the receiving system through the transmit-receive switching section.
The present invention provides a single-antenna time division control type FM-CW radar which comprises a transmitting/receiving antenna and a transmit-receive switching section and performs switching between transmission and reception by time division, wherein an amplifier is provided in each signal path in a transmitting system and a receiving system, including the transmit-receive switching section, and control means is provided for alternately causing the amplifier provided in the transmitting system and the amplifier provided in the receiving system to operate in synchronism with transmit and receive timings, thereby performing the switching between transmission and reception.
The operation of the amplifiers is controlled by controlling the gain of each of the amplifiers. The amplifiers can be made to switch on and off.
The amplifiers are connected in a multiple stage configuration, and the amplifiers connected in a multiple stage configuration are provided at a plurality of locations along each signal path in the transmitting system and the receiving system.
A multiplier is provided in series with the amplifier in a local signal separator provided in the signal path of the transmitting system, and the multiplier may be switched on and off to perform the switching between transmission and reception in conjunction with the amplifier.
A passive device can be used as the transmit-receive switching section, and a hybrid circuit or a Y-shaped power distributor can be used as the passive device.
In the single-antenna time division control type FM-CW radar of the present invention, as an amplifier is provided in each signal path in the transmitting system and the receiving system, and as the transmitting and receiving systems are isolated from each other by turning the amplifiers on and off by varying the bias voltage applied to the amplifiers, reliable isolation can be provided between the transmitting and receiving systems. Furthermore, when the amplifiers are connected in multiple stages and provided at a plurality of locations along each signal path in the transmitting system and the receiving system, isolation between the transmitting and receiving systems becomes more reliable and easier to adjust.
Further, as a passive device such as a hybrid circuit or a Y-shaped power distributor is used in the transmit-receive switching section, not only can the power consumption be reduced, but the need to use a switch in the transmit-receive switching section can be eliminated; as a result, there is no need to use ICs having different bias conditions, and the amount of bias circuitry can be reduced, achieving a cost reduction.
Furthermore, when a multiplier is used to perform switching between transmission and reception, or when an amplifier (A1, A2) is provided in front of the multiplier and operated at a low frequency band, isolation between the transmitting and receiving systems can be achieved efficiently.
By providing the isolation between the transmitting and receiving systems as described above, it becomes possible to obtain a proper receiving gain, reduce FM-AM conversion noise as well as the leakage of the switching frequency to the beat signal, and improve the S/N ratio of the receiver. This serves to improve the detection performance of the radar.